The present invention relates primarily to a method of producing a transition edge sensor, particularly for use in the analysis of radiation, along with a transition edge sensor produced according to the method.
Transition edge sensors (TES) are often used for measuring the energy of incident particles. These particles are generally photons although the energies of other kinds of particles such as thermal atoms and molecules can be measured. For this reason transition edge sensors can be found in instruments for infra-red bolometry and also for optical and X-ray spectroscopy and analysis equipment.
The primary functioning element of a TES is a region of material which undergoes a transition from conventional electrical conductivity behaviour to that of a superconductor below a specific temperature. Although this transition actually occurs over a range of temperatures, typically a transition temperature Tc is defined at the mid-point in temperature between the two alternative modes of electrical conductivity.
In general, the material is provided as a thin film on a substrate, which is then placed in thermal contact with a heat sink at cryogenic temperatures. Incident particles are absorbed either by the material directly or by an additional xe2x80x9cabsorberxe2x80x9d material placed in thermal contact with the TES. In either case the aim is to convert the energy of the incident particles into thermal energy which can then be accurately measured using the TES.
To achieve this, the thin film is maintained at a temperature within the transition region such as Tc. The absorption of a particle into the film causes a rise in the film temperature which may be detected as a corresponding increase in the film""s resistivity. The superconducting transition in such TES devices occurs at very low temperatures close to absolute zero and typically Tc may be below 300 mK.
It is desirable to obtain the maximum energy resolution possible as for example in X-ray analysis, this allows identification of the particular elemental materials from which the X-rays originate.
The energy resolution is a function of the heat capacity of the device. Very low temperatures produce the lowest heat capacity and best energy resolution results but such low temperatures often have a detrimental effect on the detection speed of the device as the thermal conductivity also decreases rapidly with decreasing temperature. Therefore at lower temperatures a slower count rate is achieved. Another factor is the cooling efficiency of the apparatus. A higher Tc makes cooling easier. In some applications the very highest energy sensitivity will be required whereas in others the count rate will be more important. Therefore the desired Tc is dependent upon the particular application.
Unfortunately only a small number of elemental superconductors exhibit a Tc at convenient temperatures. Some examples of these include cadmium (Cd), iridium (Ir) and tungsten (W).
Some examples of sensors utilising these elements are described in xe2x80x9cPhase transition thermometers with high temperature resolution for calorimetric particle detectors employing dielectric absorbersxe2x80x9d, W. Seidel et al, Physics Letters B, volume 236, number 4, Mar. 1, 1990, pages 483 to 487; and xe2x80x9cIr TES for x-ray microcalorimetersxe2x80x9d, D. Fukuda et al, Nuclear Instruments and Methods in Physics Research A, 444 (2000), pages 241 to 244.
As only a handful of superconducting elements have a convenient Tc, there is much interest in tunable transition temperatures. This has led to the development of xe2x80x9cbi-layerxe2x80x9d sensors, in which a second layer of material is deposited upon the first, one layer being a conventional conductor, thereby forming an xe2x80x9cSNxe2x80x9d bi-layer.
One example of such a bi-layer is that of aluminium (Al) and silver (Ag), the aluminium providing a layer which is superconducting and the silver layer behaving as a normal conductor. Such bi-layers have a transition temperature Tc which is different from those of single film elemental superconductors due to the xe2x80x9cproximity effectxe2x80x9d. By controlling the thickness of these two layers, Tc can be controlled.
Although the control of Tc using bi-layers is advantageous, the use of bi-layers incurs a number of disadvantages which are reflected in the performance of the device. In particular, the residual resistance ratio (RRR) is significantly reduced in such devices and this is caused by electron scattering occurring at the interface between the two materials of the bi-layer.
The RRR determines the critical current which in turn determines the dynamic range of the device. The thickness possible with a bi-layer is also limited which also limits the dynamic range.
Some examples of bi-layer TES devices are described in xe2x80x9cProximity effect in iridium-gold bilayersxe2x80x9d, U. Nagel et al, J. Appl. Phys. 76 (7), Oct. 10, 1994, pages 4262 to 4263; and xe2x80x9cRecent progress in calorimeters with transition edge thermometersxe2x80x9d, F Prxc3x6bst et al, Low Temperature Detectors for Neutrinos and Dark Matter IV, Eds. N. E. Booth and G. L. Salmon, Editions Frontieres, 1992, pages 193 to 202.
In accordance with a first aspect of the present invention we provide a method of producing a transition edge sensor, the method comprising:
depositing a sensing material upon a substrate to form a sensing layer having a transition temperature; characterised by selecting a desired transition temperature of the sensing layer; and by controlling the temperature of the substrate for the deposition process in accordance with the selected transition temperature.
We have discovered that in some cases it is possible to control Tc without the need to deposit a second conventional metal layer to form a bi-layer. By maintaining the substrate at a particular temperature prior to or during the deposition process, the resultant sensing layer is found to have a Tc that is dependent upon the substrate temperature. Preferably the temperature of the substrate is controlled during the deposition process.
The temperature of the substrate may be controlled so that it is substantially at room temperature or below room temperature. However, typically the substrate temperature is controlled to be above room temperature and therefore the method preferably further comprises heating the substrate.
Advantageously the sensing material may be deposited in a single step as this reduces the fabrication time and the process complexity in producing a TES. However, an alternative multiple step deposition process could be used.
In known bi-layer devices a further deposition step is needed following the initial deposition step. Generally this would be performed in a similar manner to the deposition of the first layer. In the present invention a second deposition step is not needed. In some cases the processing of the device is simplified, as in bi-layer devices a separate processing step may be needed to define the dimensions of each layer of the bi-layer. There is also no requirement in the present invention for the use of different etch materials in the processing of the two layers of a bi-layer device.
There is an additional advantage in that the absence of two layers ensures that there is no reaction or interdiffusion between such layers. As a result the long term stability of the device is improved.
The invention also overcomes the problems caused by the interface between the layers in bi-layer devices. Such an interface tends to scatter electrons and therefore a TES made according to the method of the present invention will have a higher residual resistance ratio (RRR).
In general, the temperature of the substrate is controlled in accordance with a predetermined relationship relating the transition temperature to the temperature of the substrate. In one example an approximately linear relationship exists between the transition temperature Tc and that of the substrate during deposition. As this is dependent upon both the substrate material and the deposited sensing material, more complex but quantifiable relationships may exist in other cases.
The temperature of the substrate may alternatively be controlled in accordance with predetermined data, for example, values of substrate temperatures and corresponding transition temperatures. A simple look-up table can be used in this case to enable selection of the appropriate substrate temperature given a desired transition temperature.
A number of superconducting materials exhibit this transition temperature effect when deposited on a substrate. In principle the substrate and the sensing material may therefore be formed from any such suitable materials. However, typically the sensing material will be a high purity elemental material which exhibits a superconducting transition at low temperatures. Examples of these include iridium, tungsten and hafnium.
Preferably however the sensing material will be iridium (Ir) and typically the substrate will be silicon nitride (Si3N4).
The present invention provides a method of producing a TES with a highly controllable Tc over a large range of temperature. In the case of iridium deposited on silicon nitride, the Tc of the sensor typically lies in the approximate range 50 to 300 mK.
In accordance with a second aspect of the present invention, we provide a transition edge sensor made in accordance with the method of the first aspect of the invention. Generally such a sensor has only a single deposited sensing layer, for example comprising iridium on a silicon nitride substrate.